Piston for swash plate type compressor

ABSTRACT

The object of the present invention is to offer not only a compressor which is compact and reduced in weight, but also its piston whose neck portion is increased in strength at a time.  
     A single-headed piston for a swash plate type compressor has a hollow head portion, a pair of engaged portions engaged with a swash plate, and a neck portion connected to the pair of the engaged portions at the outer peripheral side of the swash plate. The pair of the engaged portions sandwiches both of the outer peripheral end surfaces of the swash plate through a pair of hemispherical shoes. A pair of concave portions having a pair of concave spherical surfaces is formed on the surfaces facing each other of the pair of the engaged portions so as to receive a convex spherical surfaces of shoes slidably. The pair of concave portions have a common spherical center. The spherical center is positioned on the plane which includes the central axis of the head portion and is parallel in the extending direction of the pair of the engaged portions, and is positioned deviatedly to the opposite side (to the radially inner side of the compressor) to the neck portion from the central axis of the head portion. It is desired that the spherical center is positioned deviatedly to the radially inner side of the compressor by equal to or greater than {fraction (1/30)} of the diameter of the head portion. The thickness of the neck portion is increased, and the bending strength of the neck portion is improved. In addition, the diameter of the swash plate is to be compact. Therefore, the compressor is reduced in weight, and is improved in its durability.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a swash plate type compressor, and more particularly, to improvement of a piston for use in such a compressor.

[0002] A swash plate type compressor is employed as a refrigerant compressor for a vehicle air conditioner. A type of the compressor has a swash plate which is rotated inclinably with respect to a drive shaft, a housing including cylinder bores formed in parallel with and around the axis of the drive shaft, and pistons including a pair of engaged portions engaged with both of the outer peripheral end surfaces of the swash plate, respectively, through each shoe device, a neck portion connected to both of the engaged portions at the outer peripheral side of the swash plate, and a head portion slidably disposed within the cylinder bore.

[0003] As for a piston for use in this kind of swash plate compressor, one of the important matters is to secure the strength of the neck portion. The neck portion extends through the space between the outer peripheral surface of the swash plate and the inner peripheral surface of the housing, and is connected to the pair of engaged portions. A diameter of the inner peripheral surface of the housing needs to be as small as possible to reduce the size and the weight of the compressor. It is desired that the back surface of the neck portion is as close to the drive shaft as possible. On the other hand, a maximum discharge capacity of the swash plate compressor needs to be increased. It is effective that the rotational speed of the swash plate is raised to meet both of these requirements. In this case, however, inertia force accompanied by reciprocating movement of the piston is increased, and so the bending moment acting on the neck portion at around the top dead center of the piston is increased. Therefore, the required bending strength of the neck portion increases. There are two requirements. One is that the back surface of the neck portion is as close to the axis of the drive shaft as possible. The other is that the bending strength of the neck portion is increased. These requirements are contrary to each other. One of the important matters is how to meet these requirements which are contradictory to each other.

SUMMARY OF THE INVENTION

[0004] The object of the present invention is to offer a swash plate type compressor which can be not only compact and reduced in weight but also increased in strength of the neck portion of its piston.

[0005] To achieve the above object, the present invention has following features. A piston for use in a swash plate type compressor has a pair of engaged portions operatively engaged to both of the outer peripheral end surfaces of a swash plate through a shoe device, respectively, a neck portion connected to both the pair of engaged portions at the outer peripheral side of the swash plate, and a head portion disposed within a cylinder bore slidably. The spherical center of a pair of concave spherical surfaces of engaged portions, which are engaged with convex spherical surfaces of the shoe device, is positioned deviatedly to the drive shaft from the central axis of the head portion.

[0006] The shoe device for use in the swash plate type compressor may include a ball and a shoe having a concave spherical surface for receiving the spherical surface of the ball. Otherwise, the shoe device may include a pair of substantially hemispherical shoes. In any case, a concave spherical surface is formed on the engaged portion of the piston, and a convex spherical surface of a ball or a hemispherical shoe is received in the concave spherical surface. Conventionally, it was believed generally that a spherical center of the concave spherical surface was needed to be in the central axis of the head portion of the piston, to avoid that an eccentric load should act on the piston. In the present invention, however, the spherical center of a concave spherical surface is positioned deviatedly to the opposite side to the neck portion with respect to the central axis of the piston, or to the nearer side to the axis of the drive shaft (the inner side of the compressor). As a result, an outer diameter of the swash plate is to be reduced. When the inner diameter of the housing is predetermined, the space between the outer peripheral surface of the swash plate and inner peripheral surface of the housing is to be increased. Accordingly, the thickness of the neck portion arranged in this space (the size in the radial direction of the swash plate type compressor) is to be increased. It is much more effective to increase the thickness of the neck portion than to increase the width of the neck portion (the size in the peripheral direction of the swash plate type compressor), in order that the bending strength of the neck portion is to be increased. In addition, while the piston is prevented from being increased in weight, the strength of the neck portion is effectively increased. When the spherical center of the concave spherical surface is moved to the radially inner side of the compressor, a spherical center of the convex spherical surface of the shoe device, which is received in the concave spherical surface, is also moved to the radially inner side of the compressor. The swash plate is engaged with the shoe device, so the outer diameter of the swash plate is decreased. Therefore, the reduction in weight of the swash plate type compressor is performed. Furthermore, the value of PV, which is one of the indexes indicates the efficiency of the compressor, is to be decreased. P denotes the force by which the swash plate pushes the piston, and V denotes the peripheral rotational speed of the swash plate at the contacting point against the shoe device. As above described, when the diameter of the swash plate is decreased, the peripheral speed V is decreased. If the force P is constant, the value of PV is to be decreased. If the peripheral speed V is increased with the value of PV set as that of conventional compressor, maximum discharge capacity is to be increased.

[0007] Furthermore, the present invention has a following feature. The spherical center is positioned deviatedly to the drive shaft from the central axis of the head portion by equal to or greater than {fraction (1/30)} of the diameter of the head portion.

[0008] In this case, when the deviated value is increased, the strength of the neck portion is increased. For example, when the deviated value is {fraction (1/20)}, {fraction (1/16)}, {fraction (1/14)} or {fraction (1/12)} instead of {fraction (1/30)} of the diameter of the head portion, the strength of the neck portion is further increased as the deviated value increases.

[0009] Furthermore, the present invention has a following feature. The pair of engaged portions and the neck portion have a single head portion.

[0010] A piston, of which head portion is only at one side, is called a single-headed piston. The single-headed piston may be applied to a fixed displacement compressor whose inclination angle is fixed. However, it is especially effective for a variable displacement compressor whose discharge capacity is adjusted in accordance with the inclination angle of the swash plate. When the present invention is applied to a single-headed piston, piston is effectively operated with not only the above effect but also the characteristic effect for single-headed piston. For example, in the variable displacement swash plate type compressor, after the compressor stops, the swash plate is returned to be the minimum inclination angle from a large inclination angle state by the urging force of the urging device. At this time, the inclination angle of the swash plate is easily returned. When the compressor stops in a large discharge capacity state, the axial length of accommodated part of one of the pistons within the cylinder bore is especially short. To return the swash plate to its minimum inclination angle by the urging device, the piston needs to be pushed into the cylinder bore. Because the swash plate is inclined, the force acted on the piston by the swash plate through the shoe device has a component facing the outer periphery. The piston is receives a torsional force in the cylinder bore by the component, and the surface pressure at the neck portion side (at the radially outer side of the compressor) between the inner peripheral surface of the cylinder bore and the outer peripheral surface of the head portion of the piston is increased. Therefore, the frictional resistance is increased. Conventionally the portion of the cylinder bore at the farther side from the drive shaft was longer in the axial direction than the portion of the cylinder bore at the nearer side to the drive shaft, and the surface pressure acting on the portion at the neck portion side of the cylinder bore was reduced. According to the present invention, the point of application of the force which is acted on the piston through the shoe device is positioned deviatedly to the radially inner side of the compressor. The rotational moment caused by the deviated load is generated to the piston. The direction of the rotational moment is opposite to that of the rotational moment due to the force of the above component toward the outer periphery. Therefore, those rotational moment is to be reduced by canceling each other.

[0011] Furthermore, the present invention has a following feature. The pair of concave spherical surfaces of the engaged portions has a common spherical center.

[0012] The piston for use in the swash plate type compressor according to the present invention, is especially effective for a variable displacement compressor. When the pair of concave spherical surfaces of the engaged portions has a common spherical center, even if the inclination angle of the swash plate changes, the distance between the farthest two portions of a pair of hemispherical shoes does not change. Accordingly, the clearance between a pair of the engaged portions and the shoe device does not change. This type of the piston is to be applied to the fixed displacement compressor, too.

[0013] Furthermore, the present invention has a following feature. A back surface of the neck portion is positioned nearer to the central axis than an outer peripheral surface of the head portion of the piston.

[0014] According to the present invention, the back surface of the neck portion of the piston does not need to be machined. The head portion is, in general, to be machined so as to be disposed within the cylinder bore in high accuracy. When the back surface of the neck portion is positioned nearer to the central axis of the drive shaft than the outer peripheral surface of the head portion, the back surface does not need to be machined. Accordingly, time and process of machining is not only reduced, but also the strength of the neck portion is improved. A blank of a piston is, in general, manufactured by a forging or by a casting. The blank of the piston without machining is larger in strength than the machined piston.

[0015] In a single-headed piston, a rotation preventing member having a partially cylindrical surface whose radius of curvature is larger than the outer peripheral surface of the head portion is formed. By the engagement between the rotation preventing member and the inner peripheral surface of the housing, the single-headed piston is generally prevented from the rotation around its central axis. The outer peripheral surface of the rotation preventing member is also generally machined. However, when the back surface of the neck portion is positioned nearer to the central axis of the drive shaft than the outer peripheral surface of the rotation preventing member, the back surface of the neck portion does not need to be machined.

[0016] Furthermore, the present invention has a following feature. The end being farther from said neck portion, of an engaged portion at the head portion side is positioned within an imaginary cylindrical surface extending to include an outer peripheral surface of the head portion.

[0017] According to the present invention, even when the engaged portion at the head portion side is disposed into the cylinder bore at the top dead center of the piston, the interference between the end of the engaged portion and the inner peripheral surface of the cylinder bore is prevented.

[0018] Furthermore, the present invention has a following feature. The neck portion has at least a rib extending in the axial direction on the back surface.

[0019] According to the present invention, the bending strength of the neck portion is effectively increased while the piston is prevented from being increased in weight. A plurality of ribs are generally more effectively applied to the invention than a single rib.

[0020] Furthermore, the present invention has a following feature. A swash plate type compressor has a swash plate rotated so as to incline with respect to an axis of a drive shaft, a housing having cylinder bores formed in parallel with and around the axis of the drive shaft, a pair of engaged portions operatively engaged to both of the outer peripheral end surfaces of a swash plate through a shoe device, respectively, a neck portion connected to both the pair of engaged portions at the outer peripheral side of the swash plate, and a head portion disposed within a cylinder bore slidably. The spherical center of a pair of concave spherical surfaces of engaged portions, which are engaged with convex spherical surfaces of the shoe device, is positioned deviatedly to the drive shaft from the central axis of the head portion.

[0021] Furthermore, the present invention has a following feature. The shoe device is a pair of substantially hemispherical shoes including the convex spherical surfaces and the sliding surfaces which are substantially plane.

[0022] Furthermore, the present invention has a following feature. The compressor has a device for adjusting inclination angle of the swash plate. The pair of concave spherical surfaces of the engaged portions has a common spherical center.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

[0024]FIG. 1 is a cross-sectional view illustrating a swash plate compressor according to an embodiment of the present invention;

[0025]FIG. 2 is an elevational view illustrating a piston of the above swash plate compressor;

[0026]FIG. 3 is a cross-sectional view as seen from line I-I in FIG. 2; and

[0027]FIG. 4 is a side cross-sectional view illustrating a piston for use in a swash plate compressor according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] A swash plate compressor and its pistons for use in a vehicle air conditioner according to embodiments of the present invention will now be described, referring drawings.

[0029] The swash plate compressor according to the embodiments is shown in FIG. 1. Herein, a plurality of cylinder bores 12 extended in parallel with the central axis of a cylinder block 10 therearound are formed with an equal angle, respectively. A single-headed piston 14 is accommodated in each cylinder bore 12 to be reciprocated. A front housing 16 is secured to an end surface in the axial direction of the cylinder block 10 (which is the end surface at the left side of the cylinder block 10 in FIG. 1, or a front end surface), and a rear housing 18 is secured to another end surface (which is the end surface at the right side of the cylinder block 10 in FIG. 1, or a rear end surface) through a valve plate 20. A housing assembly for the swash plate compressor is constituted by the front housing 16, the rear housing 18 and the cylinder block 10. A suction chamber 22 and a discharge chamber 24 are formed between the rear housing 18 and the valve plate 20. Each chamber is connected through an inlet 26 and an outlet 28, respectively, to a refrigerant circuit which is not illustrated. Suction ports 32, suction valves 34, discharge ports 36, discharge valves 38 and the like are formed on the valve plate 20.

[0030] A drive shaft 44 is mounted rotatably to the central axis of the cylinder block 10. The drive shaft 44 is supported by the front housing 16 and the cylinder block 10 through bearings at both of its end portions. A central support hole 48 is formed in the central part of the cylinder block 10, and the drive shaft 44 is supported in the central support hole 48. The end portion of the drive shaft 44 at the front housing 16 side is connected to a vehicle engine as an outer drive source (not illustrated) through a clutch mechanism such as a magnetic clutch. Accordingly, when the vehicle engine operates and the drive shaft 44 is connected to the vehicle engine by the clutch mechanism, the drive shaft 44 is rotated around its axis.

[0031] A swash plate 50 is mounted to the drive shaft 44 to be movable in the axial direction and inclinable with respect to the drive shaft 44. A through hole 52 is formed in the center of the swash plate 50, and the drive shaft 44 penetrates the through hole 52. The vertical length of the through hole 52 gradually increases in the axially opposite directions from its axially intermediate portion towards the axially opposite ends. Both of the end surfaces of the through hole 52 are in the shape of a longitudinal hole.

[0032] A rotor 54 as a rotation converting member is mounted to the drive shaft 44 and is engaged with the front housing 16 through a thrust bearing 56. The swash plate 50 is integrally rotated with the drive shaft 44 by a hinge mechanism 60, and is movable in the axial direction and inclinable with respect to the drive shaft 44. The hinge mechanism 60 includes a pair of support arms 62 fixed to the rotor 54, a pair of guide pins 66 slidably fitted in each guide hole 64 of the support arms 62, the through hole 52 of the swash plate 50, and the outer circumferential surface of the drive shaft 44. In this embodiment the drive shaft 44, the hinge mechanism 60 constituting a rotation converting device and the like constitute a swash plate drive device. The swash plate drive device and the swash plate 50 constitute a drive device which reciprocates the piston 14.

[0033] The piston 14 is a kind of a hollow piston. The piston 14 includes a pair of engaged portions 70 and 72 engaged with the swash plate 50, a neck portion 74 connected to both engaged portions 70 and 72 outside the outer circumference of the swash plate 50, and a hollow head portion 76 slidably fitted in the cylinder bore 12 and formed integrally with the engaged portions 70 and 72 and the neck portion 74. The engaged portions 70 and 72, the neck portion 74, and the head portion 76 are not necessary to be integrally formed, and they may be formed by means of a connection such as a screw. The swash plate 50 is engaged with the engaged portions 70 and 72 through a pair of hemispherical shoes 78. The shapes of the piston 14 and the shoe 78 will be described in detail afterward.

[0034] The rotating movement of the swash plate 50 is converted to reciprocating movement of the piston 14 through the shoe 78. In the suction stroke, where the piston 14 moves from the top dead center to the bottom dead center, the refrigerant gas in the suction chamber 22 is drawn into the cylinder bore 12 through the suction port 32 formed on the valve plate 20, pushing away the suction valve 34. In the discharge stroke, where the piston 14 moves from the bottom dead center to the top dead center, the refrigerant gas in the cylinder bore 12 is compressed, and discharged into the discharge chamber 24 through the discharge port 36 formed on the valve plate 20, pushing away the discharge valve 38. The compressive stress accompanied by the compression of the refrigerant gas in the axial direction acts on the piston 14. The compressive stress is received by the front housing 16 through the piston 14, the swash plate 50, the rotor 54, and the thrust bearing 56.

[0035] A crank chamber 86 is defined between the front housing 16 and the cylinder block 10. A supply passage 80 is formed to penetrate the cylinder block 10, and connects the crank chamber 86 and the discharge chamber 24. A displacement control valve 90 is arranged in the supply passage 80. The displacement control valve 90 is a magnetic valve, and a solenoid 92 is excited and de-excited by a control device mainly constituted by a computer (not illustrated). The opening degree of the displacement control valve 90 is adjusted due to the supplied electric current in accordance with the data such as a cooling load.

[0036] A releasing passage 100 is formed in the drive shaft 44. The releasing passage 100 is opened to the central support hole 48 at one of its opposite ends, and to the crank chamber 86 at the other end. The central support hole 48 communicates with the suction chamber 22 through a releasing port 104.

[0037] The swash plate compressor according to the present invention is a variable displacement type. The pressure in the crank chamber 86 is adjusted due to the difference in pressure between the discharge chamber 24 as a higher pressure source and the suction chamber 22 as a lower pressure source. The difference in pressure between the crank chamber 86 and the compression chamber in the cylinder bore 12 acting on the front side and the rear side of the piston 14 is adjusted. The stroke of the piston 14 is changed in accordance with the inclination angle of the swash plate 50. Therefore, the discharge capacity of the compressor is adjusted. In detail, the crank chamber 86 is selectively connected and disconnected to and from the discharge chamber 24 due to the de-excitement and the excitement of the displacement control valve 90. Therefore, the pressure in the crank chamber 86 is adjusted. The abutment between a stopper 106 to be formed integrally with the swash plate 50 and the rotor 54 regulates the maximum inclination angle of the swash plate 50. The abutment between the swash plate 50 and a stopper 108 to be formed integrally with the drive shaft 44 regulates the minimum inclination angle of the swash plate 50. The inclination angle adjusting device, or the discharge capacity control device is constituted by the supply passage 80, the crank chamber 86, the displacement control valve 90, the releasing passage 100, the releasing port 104, the control device and the like.

[0038] A compression coil spring 110 as an elastic member, which is a kind of an urging device, is arranged around the drive shaft 44 between the swash plate 50 and the rotor 54. The swash plate 50 is urged toward the nearly perpendicular angle position to the central axis of the cylinder block 10 (the axis of the drive shaft 44) at which the swash plate 50 abuts against the stopper 108. When the compressor stops, the swash plate 50 is abutted against the stopper 108 by the urging force of the spring 110, and the swash plate 50 is to wait for restarting. A recess 112 whose diameter is larger than the other portions of the through hole 52 is formed on the end portion at the rotor 54 side of the through hole 52 of the swash plate 50. When the swash plate 50 inclines at the maximum angle, an end of the spring 110 is received on a receiving portion 114 of the recess 112 perpendicular to the central axis of the cylinder block 10. When the swash plate inclines at the minimum angle, the end of the spring 110 is received on a receiving portion 116 of the recess 112 perpendicular to the above central axis.

[0039] The cylinder block 10 and the piston 14 are made of an aluminum alloy which is a kind of metal, and the outer peripheral surface of the piston 14 is coated with fluoro resin film. When the piston 14 is coated with fluoro resin film, a direct contact between the same kinds of metal is prevented. As a result, seizure is prevented. And the amount of clearance between the coated piston 14 and the cylinder bore 12 is minimized. In this case, the following thing is desired that the cylinder block 10 and the piston 14 are made of aluminum silicon series alloy and the like. The cylinder block 10, the piston 14, the coating layer and the like may be made of the other kind of material.

[0040] The piston 14 will be described in FIGS. 2 and 3.

[0041] The head portion 76, which is not illustrated in detail, is a hollow head portion. The head portion 76 includes a cylindrical portion with a bottom portion of which the other side is opened, and a closure member which closes the opening portion of the cylindrical portion. Therefore, the weight is reduced. The cylindrical portion with the bottom portion which composes a main part of the head portion 76 is formed integrally with the engaged portion 72, by integrally forming the bottom portion and the engaged portion 72. An annular groove 122 (illustrated in FIG. 2) for retaining lubricant oil is formed on the outer peripheral surface 120 of the head portion 76 near the end surface which is opposite side to the engaged portion 72 with respect to the head portion 76.

[0042] As shown in FIG. 3, an inner surface 130 at the swash plate 50 side of the neck portion 74 of the piston 14 is plane. A back surface 132, which is opposite side to the inner surface 130 of the neck portion 74, is a partially cylindrical surface whose convex portion faces outward in a radial direction. The back surface 132 is positioned nearer to the central axis than an outer peripheral surface 120 of the head portion 76, and does not protrude radially outwardly with respect to the outer peripheral surface 120 of the head portion 76. A rotation preventing portion 136 (which is illustrated in FIG. 2) is formed integrally on the engaged portion 70. The rotation preventing portion 136 is formed integrally on the base portion side of the engaged portion 70 to which the neck portion 74 is connected. The outer peripheral surface of the rotation preventing portion 136 protrudes radially outwardly with respect to the outer cylindrical surface 120, and has a partially cylindrical surface of which radius of curvature is larger than that of the outer peripheral surface 120 of the head portion 76. Because the partially cylindrical surface of the rotation preventing portion 136 contacts an inner peripheral surface of the front housing 16, the rotation of the piston 14 around its central axis is prevented. Therefore, the collision between the piston 14 and the swash plate 50 is prevented. The end which is the opposite side to the base portion side connected to the neck portion 74, of the pair of engaged portions 70 and 72, is positioned within an imaginary cylindrical surface extending to include the outer peripheral surface 120 of the head portion 76. Therefore, the interference between the end of the engaged portions 70 and 72, and an inner peripheral surface of the cylinder bore 12 is prevented.

[0043] A pair of concave portions 140 are formed on the inner surfaces of the pair of engaged portions 70 and 72 which face each other. The inner surfaces of these concave portions 140 are substantially concave spherical surfaces. The pair of concave spherical surfaces of the pair of concave portions 140 are positioned on a respective single spherical surface. A pair of convex spherical surfaces 144 of the pair of shoes 78 is slidably received in the concave portions 140. Sliding surfaces 146, which are substantially plane, contact both of the sliding surfaces of the swash plate 50. The pair of shoes 78 sandwich both of the outer peripheral portions of the swash plate 50. The spherical center, which is the center of the pair of concave spherical surfaces of concave portions 140, is positioned on a plane, which includes the central axis of the head portion 76 of the piston 14 and is parallel in the extending direction of the pair of engaged portions 70 and 72. The spherical center is positioned deviatedly to the opposite side to the neck portion 74 side from the central axis of the head portion 76 by a predetermined distance α. The deviated value α is desired to be greater than or equal to {fraction (1/30)} of the diameter of the head portion 76. In this embodiment the deviated value is about 3 mm corresponding to {fraction (1/10)} to {fraction (1/11)} of the diameter of the head portion 76.

[0044] In this embodiment a shoe device exclusively includes a pair of hemispherical shoes 78. According to the embodiment of the present invention, the spherical center of the concave portions 140 is positioned deviatedly to the opposite side to the neck portion 74 side (to the radially inner side of the compressor) from the central axis of the head portion 76. The thickness of the neck portion 74 (the size in the direction parallel in the extending direction of the pair of engaged portions 70 and 72 of the neck portion 74) is increased. Therefore, the bending strength of the neck portion 74 is improved, and so the durability of the piston 14 is also improved.

[0045] After the compressor stopped at a large inclination angle of the swash plate 50, the swash plate 50 returns to the minimum angle easily. The swash plate 50 returns to the minimum inclination angle by urging force of the compression coil spring 110. The direction of the pushing force acted on the piston 14 from the swash plate 50 is inclined from the central axis of the piston 14. The rotation moment generates so that the back surface 132 of the neck portion 74 approaches the inner peripheral surface of the front housing 16. On the other hand, in this embodiment the spherical center of the concave portions 140 of the pair of engaged portions 70 and 72 is positioned deviatedly to the radially inner side of the compressor (or the central axis side of the compressor) from the central axis of the head portion 76. By the force acting from the swash plate 50 on the piston 14, through the shoe 78 and the concave portion 140, a rotation moment is generated on the piston 14 in the reverse direction of the above rotation moment. The above rotation moment is reduced for the cancellation by the reverse rotation moment. Accordingly, the surface pressure between the portion of the outer peripheral surface of the head portion 76 of the piston 14 at the radially outer side of the compressor, and the corresponding portion of the inner peripheral surface of the cylinder bore 12, is reduced. The inclination of the swash plate 50 easily returns to the minimum angle. In addition, the spherical center of the concave portions 140 is positioned deviatedly to the radially inner side of the compressor. The diameter of the swash plate 50 engaged with the pair of engaged portions 70 and 72 is to be reduced. The drive device to reciprocate the piston 14 containing the swash plate 50 and the rotor 54 is reduced in weight. Furthermore, the value of PV is also reduced because the diameter of the swash plate 50 is reduced as above described. That is, the durability of the compressor is improved.

[0046] The strength of the neck portion may be improved by forming a plurality of ribs on the back surface of the neck portion. One of the embodiments is shown in FIG. 4. As described in the above embodiment, a single headed-piston 200 shown in FIG. 4 also has a pair of engaged portions 202 (one of the engaged portions is illustrated in FIG. 4), a neck portion 206 which is connected to the base portions of the pair of engaged portions 202, and a hollow cylindrical head portion 208 formed integrally with the pair of engaged portions 202.

[0047] An inner surface 212 at the swash plate side of the neck portion 206, and a back surface 214 at the opposite side to the swash plate side of the neck portion 206, are plane. A plurality of ribs for reinforcement are formed integrally with the back surface 214. A plurality of axial ribs (three ribs in FIG. 4) 220, 222 and 224 are formed integrally so that the ribs are separated in the direction of the width, respectively, which is perpendicular to, both the extending direction of the engaged portions 202 and the central axis of the head portion 208, and the ribs extend in parallel with the central axis of the head portion 208.

[0048] A pair of concave portions 230 having concave spherical surfaces, of which constitution is the same as the concave portions 140, is respectively formed on the inner surfaces of the engaged portion 202 and another engaged portion extending from the neck portion, which face each other. The pair of concave portions 230 has a common spherical center. The spherical center is on the plane which has a central axis of the head portion 208 of the piston 200 and is parallel in the extending direction of the engaged portion 202, and is positioned deviatedly to the opposite side to the neck portion 206 from the central axis of the head portion 208. In the illustration an end of the engaged portion 202 is positioned within an imaginary cylindrical surface extending to include the outer peripheral surface 232 of the head portion 208 so as to be the farthest from the neck portion 206. The spherical center of the concave portions 230 is positioned more deviatedly to the drive shaft from the central axis of the head portion 208 than that of the above embodiment, and so the axial ribs 220, 222 and 224 having large height on the neck portion 206 are formed.

[0049] In this embodiment the same effect as the former embodiment is obtained because the spherical center of the concave portion 230 is positioned deviatedly. In addition, while the piston 14 is prevented from being increased in weight, the bending strength of the neck portion 74 is increased, and so the durability of the piston 14 is improved by forming the axial ribs 220, 222 and 224.

[0050] The axial ribs 220, 222 and 224 in the embodiment is one of the examples, and the shape, the size and the number of ribs may be variously changed. For example, each axial rib is formed at both ends of the neck portion in the direction of the width, or an axial rib is formed in the middle in the direction of the width.

[0051] The inner surfaces 130 and 212 of the neck portions 74 and 206 of the pistons 14 and 200 may be partially cylindrical surfaces whose convex portion faces radially outwardly.

[0052] As for the pistons 14 and 200, the closure member and the engaged portion may be formed integrally, and the opening of the cylindrical portion having the bottom, which is a main part of the head portion, may be closed by the closure member. Or a piston may be constituted by two divided head portions to be fixed each other, a head portion with an engaged portion and other head portion without it, which are divided at the middle of the axial direction of the integral head portion.

[0053] As for the construction of a swash plate type compressor, not only the above embodiment but also other constructions may be applied. For example, the displacement control valve 90 is not essential, and an opening and closing valve, which opens and closes mechanically due to the difference between the pressure in the discharge chamber 24 and the pressure in the crank chamber 86, may be applied. A magnetic control valve similar to the displacement control valve 90 may be arranged in the releasing passage 100, instead of or with the displacement control valve 90. Or an opening and closing valve, which opens and closes mechanically due to the difference between the pressure in the crank chamber 86 and the pressure in the suction chamber 22 may be applied.

[0054] The present invention may be applied to a single-headed piston for use in a fixed displacement swash plate type compressor, and to a double-headed piston having head portions on both sides of the engaged portions engaged with the swash plate.

[0055] Therefore the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims. 

What is claimed is:
 1. A piston for use in a swash plate type compressor comprising: a pair of engaged portions operatively engaged to both of the outer peripheral end surfaces of a swash plate through a shoe device, respectively; a neck portion connected to both said pair of engaged portions at the outer peripheral side of the swash plate; and a head portion disposed within a cylinder bore slidably, wherein the spherical center of a pair of concave spherical surfaces of engaged portions, which are engaged with convex spherical surfaces of said shoe device, is positioned deviatedly to the drive shaft from the central axis of said head portion.
 2. The piston for use in a swash plate type compressor according to claim 1 , wherein said spherical center is positioned deviatedly to the drive shaft from said central axis of said head portion by equal to or greater than {fraction (1/30)} of the diameter of said head portion.
 3. The piston for use in a swash plate type compressor according to claim 1 , wherein said pair of engaged portions and said neck portion have a single head portion.
 4. The piston for use in a swash plate type compressor according to claim 1 , wherein the pair of concave spherical surfaces of said engaged portions have a common spherical center.
 5. The piston for use in a swash plate type compressor according to claim 1 , wherein a back surface of said neck portion is positioned nearer to said central axis than an outer peripheral surface of said head portion of said piston.
 6. The piston for use in a swash plate type compressor according to claim 1 , wherein the end, being farther from said neck portion, of an engaged portion at said head portion side is positioned within an imaginary cylindrical surface extending to include an outer peripheral surface of said head portion.
 7. The piston for use in a swash plate type compressor according to claim 1 , wherein said neck portion has at least a rib extending in the axial direction on said back surface.
 8. A swash plate type compressor comprising: a swash plate rotated so as to incline with respect to an axis of a drive shaft; a housing having cylinder bores formed in parallel with and around the axis of the drive shaft; a pair of engaged portions operatively engaged to both of the outer peripheral end surfaces of a swash plate through a shoe device, respectively; a neck portion connected to both said pair of engaged portions at the outer peripheral side of the swash plate; and a head portion disposed within a cylinder bore slidably, wherein the spherical center of a pair of concave spherical surfaces of engaged portions, which are engaged with convex spherical surfaces of said shoe device, is positioned deviatedly to the drive shaft from the central axis of said head portion.
 9. The swash plate type compressor according to claim 8 , wherein said shoe device is a pair of substantially hemispherical shoes including said convex spherical surfaces and the sliding surfaces which are substantially plane.
 10. The swash plate type compressor according to claim 9 , further comprising a device for adjusting inclination angle of said swash plate, wherein the pair of concave spherical surfaces of said engaged portions has a common spherical center.
 11. The swash plate type compressor according to claim 8 , further comprising a device for adjusting inclination angle of said swash plate, wherein the pair of concave spherical surfaces of said engaged portions has a common spherical center. 